White Dwarfs in a Death Spiral

MINNEAPOLIS, MINNESOTA--Two stars locked in the closest binary orbit yet discovered are spiraling toward each other faster than any other known pairs of stars, according to a new x-ray study. But the cosmic waltz will be brief, at least in astronomical terms; the study suggests the stars will collide catastrophically within 500,000 years. Physicists may soon be able to detect gravitational ripples in space caused by the impending crash.

Astronomers unveiled the properties of the unusual pair 3 years ago (ScienceNOW, 14 March 2002). Both objects appear to be white dwarfs: compact, Earth-size remnants of stars like our sun that have burned up their nuclear fuel. Separated by just one-fifth the distance from Earth to the moon, the hot dwarfs race around each other every 321 seconds. But researchers could not easily determine whether the stars would converge or drift apart. To clarify that fate, observers have used telescopes to measure regular fluctuations of light from the dwarfs.

The latest such result, from NASA's Chandra X-ray Observatory, points to an ever-shrinking spiral. Astrophysicist Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Maryland, monitored the binary six times with Chandra in 2004. By timing when the x-ray signal waxed and waned, Strohmayer calculated that the 321-second orbit is speeding up by 0.0012 seconds each year. That's unprecedented for a pair of stars, Strohmayer says. The precise x-ray measurements strengthen two reports in 2004 by European astronomers, who used optical telescopes on the ground to deduce that the dwarfs are accelerating.

Einstein's theory of general relativity predicts that the dwarfs will inch together as the system loses energy in the form of gravitational waves: ripples in the fabric of space that spread away from careening bodies. According to Strohmayer, the observed rate of approach--2.5 centimeters per hour--shows that the merger will take about 500,000 years. The binary could already be "one of the brightest sources of gravitational waves in the sky," says Strohmayer, because the dwarfs are just 1600 light-years from Earth. He reported his findings here this week at a meeting of the American Astronomical Society.

A trio of spacecraft planned for launch by 2015 should detect those waves easily, says physicist L. Samuel Finn of Pennsylvania State University in University Park. Called the Laser Interferometer Space Antenna (LISA), the mission will gauge subtle back-and-forth motions of the spacecraft across 5 million kilometers--a sign of periodic gravitational warps. "This system will stick out like a sore thumb, literally within 40 minutes of turning LISA on," Finn says.